Electrical printing processes employing two fields of different strengths



Sept. 24, 1968 w. E. JOHNSON 3,402,659

ELECTRICAL PRINTING PROCESSES EMPLOYING TWO FIELDS OF DIFFERENTSTRENGTHS Filed Aug. 29, 1966 L viii 4-1 H1,- Voltqgg P G D Jaszizraf"AMPLIFIE I scluproa 64f: Colvrnol. 6 7

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INVENTOR (J/14M E JbHNjON QrraeuEKS United States Patent Oflice3,402,659 Patented Sept. 24, 1968 3,402,659 ELECTRICAL PRINTINGPROCESSES EMPLOYING TWO FIELDS OF DIFFERENT STRENGTHS William E.Johnson, Temperance, Mich., assignor to Owens-Illinois, Inc., acorporation of Ohio Continuation-impart of application Ser. No. 393,817,Aug. 31, 1964. This application Aug. 29, 1966, Ser.

9 Claims. (Cl. 101-129) ABSTRACT OF THE DISCLOSURE Electrical printingprocesses in which an electric field is employed to electrically chargeand accelerate printing powder particles from a powder supply bedthrough the image defining apertures of a screen or stencil to anarticle surface. The powder supply, screen and article surface areelectrically conductive and the electric field is applied in pulses of arelatively short time duration. During the application of the electricfield, the screen is maintained at a selected intermediate electricalpotential relative to the bed and article surface to minimize problemsof scattering and re-transfer of the powder particles.

This application is a continuation-in-part of my pending applicationSer. No. 393,817, filed Aug. 31, 1964, which is in turn acontinuation-in-part of my earlier application Ser. No. 323,409, filedNov. 13, 1963 (now abandoned).

In many instances, it is convenient to employ an arrangement in which astencil screen having an imagedefining aperture of the desired shape issupported in spaced relationship between a supply of powder and thesurface of the article to be printed. In the usual case, the supply ofpowder is supported upon an electrically conductive element, and thescreen takes the form of a fine wire mesh with the openings between themesh blocked by a coating, except in those regions constituting theimage-defining apertures.

With an arrangement of the foregoing type, an electric voltage sourcecan be connected between the electrically conductive element and screento establish an electric field which charges the powder particles on theconductive element and electrically attracts the particles toward thescreen. With an electric field of sufficient strength applied, many ofthe powder particles are accelerated to a velocity suflicient to passthrough the screen at the image-defining apertures and continue to thesurface of the article. Frequently, an electric potential is alsoestablished between the screen and article surface.

In arrangements of the general type described above, various problems ofresolution and density of the image on the article surface have beenencountered. Several effects tending to blur or reduce the imageresolution may be present, these effects in general falling under theheadings of retransfer and scattering.

Retransfer is encountered when particles striking the article surfacebecome oppositely charged and electrically repelled. Retransfer reducesthe density of the image applied to the surface and may be said toreduce the efficiency of transfer. Retransferred particles may make oneor more round trips between the supply and article surface or screen andarticle surface.

Problems of scattering or lack of image resolution can result from bothelectrical and aerodynamic effects. Where an electrically conductivescreen is employed, the elec trical lines of force are distorted by theconductive wires of the screen in a fashion such that lines of forcewhich begin on the electrically conductive plate which supports thepower supply curve toward the individual wires of the screen as theyapproach the wires. The electrically charged particles tend to followthe lines of force, and thus because of the curvature of the lines offorce, the particles deviate from a straight path, causing a scatteringof particles which results in an image with a fuzzy edge. Aerodynamiceffects occur because of the fact that an extremely large number oflightweight particles are simultaneously impelled across a relativelysmall air space, creating turbulent air currents which are primarilyeffective near the outer edges of the image area. This effect canusually be detected by an image which becomes gradually less dense nearthe edges, by virtue of the presence of a greater percentage of smallerparticles being carried by air currents toward the edge zone.

While the qualities of uniform density and high resolution are desirablein any powder transfer process, they are of special interest inconnection with multicolor printing operations. By employing a stencilwhich is spaced from both the article surface and a powder bed todetermine the image shape, the image, once applied to the article, isnot disturbed because the stencil is spaced from the article surface.Thus, by employment of a spaced stencil, the possibility exists foradvancing the article in succession to several stencils, each associatedwith an individual powder supply, and successively transferring powdersof different colors and image shapes to the article surface.

In the ordinary operation of this process, the powder layer is firmlypacked against the article surface and, in the absence of unduemechanical disturbance, will remain in its originally applied condition.Because there is no mechanical contact between the screen and articlesurface, a second layer of powder may be applied to the article surfacein adjacent or overlapping relationship to the first applied layerwithout specially treating or fixing the first layer prior toapplication of the second. The problems of retransfer, scattering orlack of resolution discussed are of especial concern in multicoloroperations, since retransfer, entirely apart from non-uniform density,can cause contamination of powder supplies by retransfer of one colorpowder from the article to a powder supply of a different color powder.Scattering or lack of resolution is much more noticeable at the junctureof image layers of contrasting colors.

It is a primary object of the present invention to provide an electricalprinting process of the type discussed above which is capable ofproducing images having a high degree of resolution and particletransfer efficiency.

It is another object of the invention to provide a method ofelectrically transferring printing powder particles across an air gap toan article surface wherein a focusing action is applied to particles intransit to the article to apply a sharply defined image shaped layer ofparticles to the article surface.

It is another object of the invention to provide methods operable toapply multicolor images of printing powder particles to an articlesurface in which the various colors are applied in succession withoutdisturbance of previously applied image portions.

It is another object of the invention to provide a method forelectrically applying successive layers or images of printing powderparticles to an article surface which does not require treating orfixing of one image portion or layer to the application of a successiveportion or layer.

Other objects and features of the invention will become apparent byreference to the following specification and to the drawing.

In the drawing:

FIGURE 1 is a schematic diagram of one form of the invention;

- 3 FIGURE 2 is a schematic diagram of another form of the invention;

FIGURE 3 is a schematic diagram illustrating the effect of an electricfield applied Only between the powder supply and stencil screen; and

FIGURE 4 is a schematic diagram illustrating the effect of an electricfield applied between supply and screen and a second electric fieldapplied between screen and article surface.

In FIGURE 1, there is disclosed a preferred embodiment of the inventionin diagrammatic form. A supply of printing powder particles indicated at10 is loosely piled in a layer of substantially uniform thickness uponthe surface of an electrically conductive plate 12. A coated wire meshstencil screen 14 is supported in spaced relationship above the surfaceof powder supply 10 and in spaced relationship below the surface of anarticle to be decorated, illustrated schematically at 16.

Stencil screen 14 takes the form of a relatively fine mesh steel wirewhich is covered with a coating which completely fills the openings ofthe screen mesh. To form a stencil, the coating is removed from thedesired imagedefining portions of the steel mesh to form image-definingapertures, as at 18, of the desired shape. The procedure and materialsneeded for preparation of such a stencil are by a well knownphotographic or photosensitive process, see, for example, United StatesPatent No. 3,100,150. The openings which define the image are crossed bythe wire mesh of the screen and, as illustrated by the example givenbelow, particles of printing powder can pass through the openings of thescreen from which the coating has been removed, and thus impinge uponthe surface of article 16 in areas corresponding in size and shape tothe imagedefining apertures, such as 18, in the stencil screen. Theportions of screen 14 which remain coated, of course, block the passageof particles from supply 10 to the article surface 16.

Transfer of particles from supply 10 to the surface of article 16 isaccomplished by applying electric potentials to plate 12, screen 14 andarticle 16 to establish an electric field which is operable toelectrically charge particles in supply 10 by virtue of theirassociation with plate 12, and electrically impel the particles upwardlyfrom supply 10 through openings 18 in screen 14 to the lower surface ofarticle 16.

Many compositions of printing powder or powdered ink suitable for use inthe arrangement of FIGURE 1 are presently commercially available. Theexact composition of the powder employed is dependent upon the desiredcolor, characteristics of the surface to be printed, desired imagethickness or density, the type of decoration or printing desired, theelectrical properties, and other variable factors. The powder employedmust, of course, be capable of being electrically charged and of aparticle size such that it can easily pass through the mesh of thestencil screen.

In an exemplary operation of the process illustrated in FIGURE 1, aloose layer approximately of an inch thick of a glass frit powder wassupported upon plate 12. The frit particles were of a particle sizerange between 2 and 10 microns, as measured by a Coulter counter orother standard particle size measuring technique. The size range of 2 to10 microns was preferable when used with the specific screen sizedescribed below, although an acceptable size range could have extendedfrom 1 to 50 microns.

The electrical properties of the powder are most conveniently referencedin terms of its electrical resistivity. In the particular example, theresistivity of the powder was between 10 and ohm centimeters, asmeasured by a standard cylindrical cell technique. In measuringresistivity by the standard cylindrical cell technique, the powder to bemeasured is poured into a cylindrical container of electrical insulatingmaterial to form an unpacked cylindrical body of known cross sectionalarea and axial length. The unpacked powder is then compressed byreducing its axial length by a known amount and the electricalresistance of the packed cylindrical powder body is then measured byapplying an electrical potential across its axial length.

Powder resistivities of between 10 to 10 ohm centimeters, as measured bythe above technique, have proven satisfactory. Powders having a higherresistivity may require precharging by corona or triboelectrificationtechniques, in which case the polarity of the electric field must beoriented in accordance with the polarity of the charge on the particles.

With a printing powder having the foregoing characteristics, a stencilscreen 14 of #200 mesh steel wire was employed. The openings in a #200mesh screen are of the order of fifty times or more the size ofparticles within the size range referred to above. It is desirable thatthe screen mesh be larger than the maximum particle size by an order ofbetween 10 and 100, so that bridging or clogging of the mesh openings isavoided. Within the foregoing limitations, a relatively fine mesh isdesirable, in order to achieve as uniform a field density as possibleover the area of the image apertures.

The spacing between plate 12 and screen 14, and between screen 14 andthe article surface 16, will usually be of the order of A to A of aninch and 0.002 to 0.060 of an inch, respectively. The precise spacing iscritical primarily in its relationship to the electric potential appliedacross the spacing, in that the electric field strength is of primaryconcern, and electric field strength is measured in volts per unitdistance of spacing. For uniformity of the electric field, it isnecessary that the spacing be uniform.

As indicated in the electric diagram of FIGURE 1, relatively highvoltage is applied across the space between plate 12 and screen 14,while the article surface 16 is held at a relatively low voltage withrespect to the screen, the potentials at plate 12 and article 16 beingof opposite polarity. In the specific example partially described above,the output of the respective high voltage and low voltage suppliesschematically shown in FIGURE 1 are such that an electric field having afield strength of between 60 and volts per mil exists between plate 12.and screen 14 and an electric field of between 10 and 25 volts per milexists between screen 14 and article 16.

Experience has shown that good quality transfer of powder from supply 10to article 16 in the indicated arrangement can be obtained using eithersynchronized AC or DC voltage sources, but a DC source is preferred. Themovement of charged particles from plate '12 to the surface of article16 is analogous to a flow of electric current, in that each particlereceives an electric charge from plate 12 and carries this charge to thearticle 16. Where a powder bed approximately 4 inches square isemployed, the voltage source should have sufficient power to supply atleast 'rnilliampere of current during the powder transfer, this currentbeing occasioned by the flow of charged particles described above.Satisfactory images are achieved by applying a potential pulse throughthe indicated circuitry of FIGURE 1 for a time duration of approximately25 to 300 milliseconds. The longer the pulse, the thicker the imagineapplied to article 16.

A convenient arrangement for applying pulses is illustrated in FIGURE 1in which a controlled gate connects the output of an oscillator to apower amplifier for the desired pulse interval. The ouput of the poweramplifier is fed into a high voltage transformer and rectifier whoseoutput in turn is connected across plate 12 and screen 14. When a DCvoltage is applied, it is applied in the form of a pulsating DC, inorder to pass through the transformer.

In the disclosed arrangement of FIGURE, 1, good quality transfer ofpowder particles has been achieved with electric field strengths of themagnitudes described above. A good quality transfer may be defined asone in which the powder particles are packed in a thin layer of uniformdensity on the surface of article 16 with a high degree of r,esolution,or image sharpness. In order to achieve thisresult, retransfer. andscattering of particles during the transfer must be minimized.

Retransfer is, believed to occur when a powder particle which, for thesake of example, will be said to be positively-charged,- contacts anarticle surface which is itself charged to a negative potential.Uponcontact with the negatively. charged surface, the particle mayloseits positive charge and become. negatively charged and thuselectrically'repelledfrom the negatively charged surface. Experience hasshownthat under. certain conditions, multiple transfers can occur withina relatively short time interval in which a particle may travel back andforth between supply and article -16 several times. The reversal ofcharge upon the particle upon contact with a charged surface does notnecessarily occur instantaneously and, under some conditions, it hasbeen observed that the image on the article surface is more dense duringearly stages of the transfer than in the later stages.

In an attempt to minimize the retransfer problem, the arrangement ofFIGURE 1 has been tried with the low voltage supply to the article 16disconnected. Disconnecting article surface 16 from a voltage sourceresults in an electric field configuration of the type schematicallyillustrated in FIGURE 3, in which the electric lines of force extendfrom the surface of plate 12 to the individual wires 14a of screen 14 inthe manner indicated at B. By applying a potential difference betweenplate 12 and wires 14a of sufficient magnitude, powder particles restingupon plate 12 can be accelerated to velocities high enough so that theirmomentum carries them clear and beyond the screen to the articlesurface.

With the electric field existing only between plate 12 and screen 14,scattering and lack of resolution in the resultant image on article 16are observed. This is believed to be due to at least two effects, one ofwhich is schematically indicated in FIGURE 3. Because the electric linesof force curve toward the individual wires 14a of the screen, theparticles do not travel in straight-line motion, but follow a curvedpath such as a typical path illustrated by the dotted line P in FIGURE3, thus, resulting in fuzziness along the image edges.

A second effect detrimental to image resolution where only aplate-to-screen field is applied occurs because the lack of positiveguidance of particles in transit between the screen and article surfacemakes the particles susceptible to turbulent air currents generated bythe simultaneous transit of a large number of particles through a givenair space.

When, in addition to the establishment of an electric field betweenplate 12 and screen 14, a second field is established between screen 14and the article surface, an electric field configuration somewhatsimilar to that schematically illustrated in FIGURE 4 exists. The firstelectric field extends between plate 12 and the individual wires 14a ofthe screen, as in the FIGURE 3 arrangement, the electric lines of forceof the first field being indicated at E1 in FIGURE 4, while a secondseries of lines of force E2 extend from the individual Wires 14a to thearticle surface.

A typical particle path is illustrated in dotted lines at P1 in FIGURE4, and it will be observed that, although the particle is deflectedsomewhat toward the nearest screen wire 14a as it leaves the lowerfield, the orientation of the lines of force E2 of the upper field tendto deflect the particle back toward vertical alignment with the pointfrom which it left plate 12.

The provision of an upper field between the screen and article surfaceis thus desirable to counteract the field distortion present in thelower field by virtue of the ar-. rangement of the individual screenwires 14a, and to tounteract the aerodynamic effects present. At thesame time, it is desirable to keep article surface 16 at a relativelylow potential in order to minimize retransfer effects. v v

It has been found that the most efficient method of transferringparticles from a supply such as 10 in FIG- URE 1, isto apply electricfieldsbothabove and below screen 14 to perform t-woseparate functions.The lower field between plate -12 and screen 14 isassigned thefunctionof electrically charging and accelerating powder particlesfromsupply 10 upwardly. through the image apertures 18 of screen 14, while.the field between screen 14 and article 16 is assigned the function ofmerely guiding or focusing the particles upon the article surface.

To perform their respective functions, the lower or particleaccelerating field is preferably made of as high a field strength aspossible without reaching the breakdown point of the dielectricmedium-air in the usual case. It has been found that breakdown, orsparking, in an air environment begins to occur at field strengths overvolts per mil. For transfer of a glass frit, such as in the specificexample described above, a minimum or threshold field strength of thefield between plate 12 and screen 14 of 30 to 35 volts per mil has beenfound to be the minimum strength necessary for satisfactory operation.

The lower or accelerating field strength should be sufficient toelectrically charge particles in the supply and impel the particlesthrough the screen with sufficient velocity to impinge on the articlesurface, even in the absence of an upper field.

Experience has shown that upper field strengths between screen 14 andthe article surface over 25 volts per mil result in an undesirableamount of retransfer, and the upper field strength is preferablymaintained below this figure.

The upper field strength should be as high as possible to enable thisfield to perform its particle guiding or focusing function without beingso high as to cause retransfer problems.

In FIGURE 2, an alternative form of the invention is disclosed which, inessence, differs from the FIGURE 1 embodiment in that the electricalfocusing action referred to above is further augmented by the employmentof a second mechanical focusing or collimating screen 20 which islocated between an electrically conductive screen 22 and the article 24being decorated. As in the previous case, an electrically conductiveplate 26 is employed to support a supply of printing powder particlesindicated at 28, the supply, screens and articles all being maintainedin a spaced relationship to each other. A common, high voltage source isconnected across article 24, screen 22 and plate 26 by variable voltagedividing resistances R1 and R2 which may be adjusted as desired toproportion the voltage drop between the plate and screen, and betweenthe screen and article.

Collimating or focusing screen 20 is preferably of a non-conductivematerial and, in fact, may consist of a sheet of cardboard or the likehaving openings 29 cut to match the image-defining apertures 30 of theconductive screen.

It will be noted that with the electrical connections of FIGURE 2, therespective voltage drops on either side of screen 22 are proportionaland that the total voltage drop between plates 26 and article 24 isconstant. By proportioning the voltage division, the potential of screen22 relative to the plate and article surface can be adjusted and therespective electric field strength regulated by varying the spacingbetween the various elements.

While various embodiments of the invention have been described, it willbe apparent to those skilled in the art that the disclosed embodimentsmay be modified. Therefore, the foregoing description is to beconsidered exemplary rather than limiting, and the true scope of theinvention is that defined in the following claims.

I claim:

1. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of supporting abed of powder particles capable of being electrically charged at oneside of an electrically conductive element, locating in spacedrelationship from the opposite side of said bed of particles anelectrically conductive wire mesh screen having a coated first portionwherein the mesh openings are filled with said coating and an uncoatedsecond portion having a shape corresponding to the predetermined patternof the powder layer to be applied to the article surface, locating thearticle surface to which the powder layer is to be applied in spacedrelationship from said screen at the side of said screen remote fromsaid bed, applying a first electric field between said conductiveelement and said wire mesh screen having a field strength sufiicient toelectrically charge and impel particles from said supply through theuncoated portion of said wire mesh screen to said article surface, andsimultaneously applying a second electric field between said wire meshscreen and said article surface having a field strength substantiallyless than the field strength of said first electric field to exert aguiding action on particles in transit between said screen and articlesurface to confine the areas of impingement of said particles on saidarticle surface to areas corresponding to the uncoated second portion ofsaid wire mesh screen.

2. The method as defined in claim 1 wherein the field strength of saidfirst electric field is at least 35 voltsper mil and the field strengthof said second electric field is less than 25 volts per mil.

3. The method as defined in claim 2 wherein said first electric field isapplied by connecting a source of electric potential between saidconductive element and said screen for a period of time within the rangeof 25 and 300 milliseconds.

4. The method as defined in claim 1 wherein the bed of powder particlesis supported vertically above the electrically conductive element, andthe electrically conductive wire mesh screen is vertically spaced abovesaid bed of powder.

5. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of supporting abed of powder particles capable of being electrically charged above anelectrically conductive element, locating in spaced relationship abovesaid bed of particles an electrically conductive wire mesh screen havinga coated first portion wherein the mesh openings are filled with saidcoating and an uncoated second portion having a shape corresponding tothe predetermined pattern of the powder layer to be applied to thearticle surface, locating the article surface to which the powder layeris to be applied in spaced relationship above said screen, applying afirst electric field between said conductive element and said wire meshscreen having a field strength sufficient to electrically charge andimpel particles from said supply through the uncoated portion of saidwire mesh screen to said article surface, and simultaneously applying asecond electric field between said wire mesh screen and said articlesurface having a field strength substantially less than the fieldstrength of said first electric field to exert a guiding action onparticles in transit between said screen and article surface to confinetheareas of impingement of said particles on said article surface toareas corresponding to the uncoated second portion of said wire meshscreen.

6. The method as defined in claim 5 whereinthe field strength of saidfirst electric field is at least 35 volts per mil and the field strengthof said second electric field is less than 25 volts per mil.

7. The method as defined in claim 6 wherein said first electric field isapplied by connecting a source of electric potential between saidconductive element and said screen for a period of time within the rangeof 25 and 300 milliseconds.

8. The method of applying a layer of powder particles to an articlesurface in a predetermined pattern comprising the steps of: supporting abed of powder particles capable of being electrically charged by anelectrically conductive element, located in spaced relationship to saidbed of particles an electrically conductive wire mesh screen having acoated first portion wherein the mesh openings are filled with saidcoating and an uncoated second portion having a shape corresponding tothe predetermined pattern of the powder layer to be applied to thearticle surface, locating the article surface to which the powder layeris to be applied in spaced relationship with said screen, applying afirst electric field between said conductive element and said wire meshscreen'having a field strength sufficient to electrically charge andimpel particles from said supply through the uncoated portion of saidwire mesh screen to said article surface, and simultaneously applying asecond electric field between said wire mesh screen and said articlesurface having a field strength substantially less than the fieldstrength of said first electric field to exert a guiding action onparticles in transit between said screen and article surface to confinethe areas of impingement of said particles on said article surface toareas corresponding to the uncoated second portion of said wire meshscreen.

9. The method as defined in claim 8 wherein the field strength of saidfirst electric field is at least 35 volts per mil and the field strengthof said second electric field is less than 25 volts per mil, and saidfirst electric field 1s applied by connecting a source of electricpotential between said conductive element and said screen for a periodof time within the range of 25 and 300 milliseconds.

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ROBERT E. PULFREY, Primary Examiner. E. S. BURR, Assistant Examiner.

